The present invention relates generally to rotatable members that are able to achieve balanced conditions throughout a range of rotational speeds. The present invention also relates to methods and systems for dynamically balancing rotatable members through the continual determination of out-of-balance forces and motion to thereby take corresponding counter balancing action. The present invention additionally relates to methods and systems for dynamically identifying resonance in rotating systems.
Mass unbalance in rotating machinery leads to machine vibrations that are synchronous with the rotational speed. These vibrations can lead to excessive wear and to unacceptable levels of noise. Typical imbalances in large, rotating machines are on the order of one inch-pound.
It is a common practice to balance a rotatable body by adjusting a distribution of moveable inertial masses attached to the body. This state of balance may remain until there is a disturbance to the system. A tire, for instance, can be balanced once by applying weights to it. This balanced condition will remain until the tire hits a very big bump or the weights are removed. However, certain types of bodies that have been balanced in this fashion will generally remain in balance only for a limited range of rotational velocities. A centrifuge for fluid extraction, however, can change the amount of balance as more fluid is extracted.
Many machines are also configured as freestanding spring mass systems in which different components thereof pass through resonance ranges during which the machine may become out-of-balance. Additionally, such machines may include a rotating body loosely coupled to the end of a flexible shaft rather than fixed to the shaft as in the case of a tire. Thus, moments about a bearing shaft may also be created merely by the weight of the shaft. A flexible shaft rotating at speeds above half of its first critical speed can generally assume significant deformations, which add to the imbalance. This often poses problems in the operation of large turbines and turbo generators.
Machines of this kind usually operate above their first critical speed. As a consequence, machines that are initially balanced at relatively low speeds may tend to vibrate excessively as they approach full operating speed. Additionally, if one balances to an acceptable level rather than to a perfect condition (which is difficult to measure), the small remaining out-of-balance will progressively apply increased force as the speed increases. This increase in force is generally due to the fact that F is proportional to rxcfx892, (where F represents the out-of-balance force, r represents the radius of the rotating body and xcfx89 represents its rotational speed).
The mass unbalance distributed along the length of a rotating body gives rise to a rotating force vector at each of the bearings that support the body. In general, the force vectors at respective bearings are not in phase. At each bearing, the rotating force vector is generally opposed by a rotating reaction force, which may be transmitted to the bearing supports as noise and vibration. The purpose of active, dynamic balancing is generally to shift an inertial mass to the appropriate radial eccentricity and angular position for canceling the net unbalance. At the appropriate radial and angular distribution, the inertial mass will generate a rotating centrifugal force vector equal in magnitude and phase to the reaction force referred to above. Although rotatable objects find use in many different applications, one particular application is a rotating drum of a washing machine.
Many different types of balancing schemes are known to those skilled in the art. U.S. Pat. No. 5,561,993, which was issued to Elgersma et al. on Oct. 22, 1996, discloses a self-balancing rotatable apparatus. U.S. Pat. No. 5,561,993 is incorporated by reference. Elgersma et al. disclosed a method and system for measuring forces and motion via accelerations at various locations in a system. The forces and moments were balanced through the use of a matrix manipulation technique for determining appropriate counterbalance forces located at two axial positions of the rotatable member. The method and system described in Elgersma et al. accounted for possible accelerations of a machine, such as a washing machine, which could not otherwise be accomplished if the motion of the machine were not measured. Such a method and system was operable in association with machines that are not rigidly attached to immovable objects, such as concrete floors. Thus, the algorithm disclosed by Elgersma et al. permitted counterbalance forces to be calculated even though a washing machine can be located on a flexible or mobile floor structure combined with carpet and padding between the rotating machinery and a rigid support structure.
Convergence problems have been noted, however, in prototype implementations of U.S. Pat. No. 5,561,993. Such convergence problems are associated with resonant frequencies of prototype implementations. In particular regions of operation surrounding the resonant frequencies, the response of the system to inputs (i.e. test or control actions) can be greatly amplified. This creates problems because in the control system being used the creation of each control model relies on performing test actions. These test actions are performed at each speed and the size of the action can be predetermined based on applying a known force at each incremental speed of operation. As the machine enters the region surrounding the resonance, these test actions may push the machine very far from balanced conditions due to the amplifying effect. If the test actions drive the machine too far up a resonant peak, or too far away from balanced conditions, an associated control algorithm may not converge quickly or may be unable to drive the motion of the prototype below acceptable limits.
As described herein, the size of the test action may be based on the amount of force that is generally required to permit the sensor measurements associated with the rotating machine to change (respond) by a meaningful amount. In regions surrounding the resonance, a smaller test action may produce this meaningful response and may also serve to hold the system closer to its balanced state.
In some systems one could merely characterize the frequency response of the system and then use the characterization to identify the speeds that correspond to resonance and the surrounding regions. Based on these tests one could then adjust the test action magnitudes accordingly in these regions. However, in some situations such as those described in the preferred embodiment of U.S. Pat. No. 5,561,993, the system in question may be dynamic. For example, in a washing machine application the location and characteristics of the out-of-balance changes with each load of clothes. In the preferred embodiment of the present invention, the frequency response of the machine can be highly related to the mass of the spinner out-of-balance (i.e., eccentric mass), and, as a result, the RPM associated with the resonance also changes with each new wash load or machine run.
Based on the foregoing, it can be appreciated that previous methods for dynamically balancing a rotatable member have experienced severe limitations in the degree of balance that can be achieved and in the rotational speeds under which they are workable. In order to overcome some of the limitations of these inventions it may be desirable to dynamically identify the resonance associated with self-balancing rotatable devices, such as the system disclosed in U.S. Pat. No. 5,561,993. The ability to dynamically identify resonance can permit the size of test actions to be adjusted to mitigate the amplification of a machine or system""s response to test actions. The incremental changes in speed in regions surrounding the resonance frequencies of rotating machinery can also be adjusted to compensate for the amplified sensor responses that result from the increase in force associated with an increase in rotational speed.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
It is, therefore, one aspect of the present invention to provide methods and systems in which rotatable members can achieve balanced conditions throughout a range of rotational speeds.
It is another aspect of the present invention to identify resonance in a rotating system and use this information to adjust control actions and speed change increments to facilitate the achievement of balanced conditions throughout a range of rotational speeds.
It is yet another aspect of the present invention to determine if a rotating system is entering resonance by comparing predicted sensor readings to current sensor readings.
The above and other aspects can be achieved as is now described. A method and system for identifying resonance in a rotating system having at least one sensor therein are disclosed. As the speed of the system is changed, a change in sensor readings may occur due to the changed speed of the rotating system. If a reading from at least one sensor within the rotating system is obtained prior to a speed change, one can calculate predicted sensor readings for a new speed for at least one sensor within the rotating system. The predicted readings may be calculated based on a control model of the rotating system and the desired change in the speed of the rotating system. These predicted sensor readings serve as a prediction of the system response to a speed change based on a system that is not operating within its natural frequency band. Then, after the actual change in speed has been carried out, new sensor readings may be obtained from at least one sensor. These actual sensor readings can then be compared to the predicted sensor readings to determine if the rotating system is entering resonance, based on a difference between the current sensor readings and the predicted sensor readings.
Once a region of resonance has been identified, at least one test action may be adjusted to mitigate the amplification of sensor measurement responses of the rotating system. The change between new speeds may also be altered in response to identification of a resonant condition. The control model may then be constructed using the modified test actions and then used to determine the control actions that are required to drive the sensor measurement response of the rotating system to a balanced condition. The rotating system may be balanced by applying control actions to the system determined utilizing the control model.